The U.S. Navy sent a guided-missile destroyer close to China's man-made islands in the disputed South China Sea on Tuesday, drawing an angry rebuke from Beijing, which said it warned and followed the American vessel.
The patrol by the USS Lassen was the most significant U.S. challenge yet to 12-nautical-mile territorial limits China asserts around the islands in the Spratly archipelago and could ratchet up tensions in one of the world's busiest sea lanes.
One U.S. defense official said the USS Lassen sailed within 12 nautical miles of Subi Reef. A second defense official said the mission, which lasted a few hours, also included Mischief Reef and would be the first in a series of freedom-of-navigation exercises aimed at testing China's territorial claims.
China's Foreign Ministry said the "relevant authorities" monitored, followed and warned the USS Lassen as it "illegally" entered waters near islands and reefs in the Spratlys without the Chinese government's permission.
"China will resolutely respond to any country's deliberate provocations. We will continue to closely monitor the relevant seas and airspace, and take all necessary steps in accordance with the need," the ministry said in a statement that gave no details on precisely where the U.S. ship sailed.
"China strongly urges the U.S. side to conscientiously handle China's serious representations, immediately correct its mistake and not take any dangerous or provocative acts that threaten China's sovereignty and security interests," it said.
The second U.S. defense official said additional patrols would follow in the coming weeks and could also be conducted around features that Vietnam and the Philippines have built up in the Spratlys.
"This is something that will be a regular occurrence, not a one-off event," said the official. "It's not something that's unique to China."
White House spokesman Josh Earnest referred questions on any specific operations to the Pentagon but said the United States had made clear to China the importance of free flow of commerce in the South China Sea.
The United States had not conducted a patrol within 12 miles of the seven Chinese outposts since Beijing began building the reefs up at the end of 2013. The U.S. Navy last went within 12 miles of Chinese-claimed territory in the Spratlys in 2012.
China claims most of the South China Sea, through which more than $5 trillion of world trade passes every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.
Risk of escalation
The decision to go ahead with the patrol follows months of deliberation and risks upsetting already strained ties with China.
"By using a guided-missile destroyer, rather than smaller vessels ... they are sending a strong message," said Ian Storey, a South China Sea expert at Singapore's Institute of South East Asian Studies.
"They have also said, significantly, that there will be more patrols - so it really now is up to China how it will respond."
Some experts have said China would likely resist attempts to make such U.S. actions routine. China's navy could for example try to block or attempt to surround U.S. vessels, they said, risking an escalation.
Euan Graham, director of the International Security Program at the Lowy Institute in Sydney, said while there was likely to be a strong vocal reaction from China, its military response could be muted.
The patrol could prompt China to do more to exert its sovereignty in the region through further reclamations and greater militarization, he added.
Both Subi and Mischief Reefs were submerged at high tide before China began a massive dredging project to turn them into islands in 2014.
Under the U.N. Convention on the Law of the Sea, 12-nautical mile limits cannot be set around man-made islands built on previously submerged reefs.
Washington worries that China has built up its outposts with the aim of extending its military reach in the South China Sea. China says they will have mainly civilian uses as well as undefined defense purposes.
The patrol comes just weeks ahead of a series of Asia-Pacific summits President Barack Obama and Chinese President Xi Jinping are expected to attend in the second half of November.
Xi surprised U.S. officials after a meeting with Obama in Washington last month by saying that China had "no intention to militarize" the islands.
Even before that, however, satellite photographs had shown the construction of three military-length airstrips by China in the Spratlys, including one each on Subi and Mischief reefs.
Some U.S. officials have said that the plan for patrols was aimed in part at testing Xi's statement on militarization.
In May, the Chinese navy issued eight warnings to the crew of a U.S. surveillance aircraft that flew near the artificial islands but not within the 12-mile limit, reported CNN, which was aboard the U.S. aircraft.
Pentagon officials say the United States regularly conducts freedom-of-navigation operations around the world to challenge excessive maritime claims.
In early September, China sent naval vessels within 12 miles of the Aleutian Islands off Alaska. China said they were there as part of a routine drill following exercises with Russia.